(a) Field of the Invention:
The present invention relates to a photographing zoom lens system and more specifically to a very compact zoom lens system comprising a small number of lens elements and suited for use in photographing cameras equipped with electronic pickup tubes and solid-state image sensors as photographing devices.
(b) Description of the Prior Art:
The lens system designed for use in photographing cameras are mostly zoom lens systems.
Most of these zoom lens systems have aperture ratios of F/2.8 or higher and zoom ratios of 3 or higher. Speaking of the fundamental composition, these zoom lens systems generally comprise, in the order from the object side, a first lens unit having positive refractive power, fixed for varying focal lengths and movable for focusing the zoom lens system, a second lens unit having negative refractive power, movable for varying focal lengths and fixed for focusing the zoom lens systems, a third lens unit having negative refractive power, moved for compensating the deviation of the image positions caused by varying focal lengths and fixed for focusing the zoom lens systems, and a fourth lens unit consisting of a positive subunit having a function to make nearly afocal the light remarkably diverged in the third lens unit and always kept fixed.
In a zoom lens system of the type described above, it is general that the first lens unit comprises three lens elements, the second lens unit comprises three lens elements, the third lens unit comprises a single lens element and the fourth lens unit comprises six to eight lens elements. The reason for selecting such a lens composition is that either of the first and second lens units requires three lens elements to correct chromatic aberration, and the fourth lens unit requires at least six lens elements to correct spherical aberration in addition to chromatic aberration. For example, the zoom lens system invented by the inventor of the present invention and disclosed by Japanese Published Unexamined patent application No. 184220/60 is designed for a zoom ratio of 3 and an aperture ratio of F/2.0, and comprises thirteen lens elements in total.
Conventionally, the zoom lens systems for photographing cameras using the solid-state image sensors, etc. are more excellent in aperture ratios and zoom ratios than the zoom lens systems for still cameras using 35 mm films of rica size when both the types of zoom lens systems are designed for the same field angle. However, this excellence is traced to the fact that short focal lengths are sufficient for the electronic photographing devices having small image planes. In other words, the performance of the above-mentioned zoom lens system is obtained by prolonging total lengths of the zoom lens systems and using a large number of lens elements in the lens systems. Therefore, such a design is against the demands for the electronic photographing cameras (Video movie cameras and still video cameras), i.e., compactness and low manufacturing cost.